How to stop the overflow siphon from draining the water tank
After the evil siphon starts, open tank main drain valve to force the tank to drain out the main drain hose instead of the overflow hose, then wait for the "Glug Glug" sound (about 30 seconds) then close the main drain valve. The draining force of the main drain valve will overpower the drain force of the overflow hose (the one that will start a siphon and drain most of your water tank unless you take these steps to stop it). This causes the overflow hose to flow in the reverse direction, which means start sucking air and stop dribbling water, which effectively breaks the siphon. Now you can close your main tank drain valve. You will have only lost 1 or 1.5 gallons - not enough to worry about. Your tank will still be so full that a little water may splash out on the highway with road bumps.
I would consider this an undocumented or poorly-documented procedure rather than a design defect.